Thermoelectric properties of half-Heusler alloys Zr1−xYxNiSn1−ySby

doi:10.1016/j.jallcom.2006.02.075

Journal of Alloys and Compounds

Volume 428, Issues 1–2, 31 January 2007, Pages 262-267

https://doi.org/10.1016/j.jallcom.2006.02.075 Get rights and content

Abstract

Half-Heusler ZrNiSn alloys, where Zr or Sn site is substituted by Y or Sb, respectively, were synthesized and their thermoelectric properties were investigated. Such substitution much affects the thermoelectric properties of ZrNiSn by changing the valence electron count (VEC) of the material. The behaviors in the thermoelectric properties for Zr_1−xY_xNiSn or ZrNiSn_1−ySb_y system can be understood as the result of the increase in the hole or electron carrier with an increase of x or y, respectively. The substitution for Zr site by Y in ZrNiSn is effective for the reduction of the thermal conductivity, while the substitution for Sn site by Sb in ZrNiSn is effective for the reduction of the electrical resistivity. We have also synthesized the double-substituted alloys, Zr_1−xY_xNiSn_1−ySb_y. The conducting property of Zr_1−xY_xNiSn_1−ySb_y is mainly dominated by the content of Sb. The phonon scattering is almost affected by the total contents of the substituent regardless of the substituted site or element. As a result, ZrNiSn_0.98Sb_0.02 shows a maximum ZT of 0.28 at 573 K in this investigated Zr_1−xY_xNiSn_1−ySb_y system.

Introduction

Half-Heusler alloys MNiSn, where M = Ti, Zr or Hf, have been studied by many researchers for the candidate as the practical thermoelectric materials because of their good thermoelectric performance and low toxicity of the constituent elements [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. These alloys are crystallized in MgAgAs-type structure, i.e., defective Heusler alloys [19]. Heusler alloys with a formula of AT₂X is a stuffed version of sodium chloride type, where the large atom (A) and the metalloid (X) are in NaCl arrangement, and the transition metal (T) fills all eight of the eight-coordinated interstitial positions per unit cell. In the half-Heusler alloys, only half of those interstitial positions are filled with transition metals.

The narrow band gap of 0.1–0.2 eV is found in these half-Heusler alloys [1], [5] and the conduction properties of them are much affected by the valence electron count (VEC) [2], [3], [4]. For the phases with VEC of 18, the position of the Fermi level E_F lies at the top of the highest occupied valence band, and so such phases show the semiconducting property and have the moderate Seebeck coefficient (S) and electrical resistivity (ρ) as the thermoelectric material. When the VEC is smaller or larger than 18, the phase ideally shows a p-type or n-type metallic conducting property, respectively. It is possible to control the VEC of MNiSn by the substitution for the M, Ni or Sn site by other elements. For example, the calculative VEC for MNi_1−xCo_xSn, where the Ni site is partially substituted by Co atom, is 18 − x. Up to now, such substitution effects have been investigated by many researchers [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].

On the other hand, the thermal conductivity of these half-Heusler alloys is generally too large to achieve a reasonable figure of merit. The total thermal conductivity κ_total of the material consists of κ_ph and κ_car, where κ_ph is the lattice contribution and κ_car is the carrier contribution to the thermal conductivity. κ_car generally increases with an increase of the number of the carrier in the material, while κ_ph is reduced when the phonon is scattered by the disorder in the material, for example, the grain boundary, the precipitated impurities or the substituted atoms. The degree of the phonon scattering depends on the density of the disorder; the more the disorder exists in the material, the more strongly the phonon is scattered. And so, the materials with more substituted atoms generally posses lower thermal conductivity. In this study, we have synthesized the half-Heusler ZrNiSn alloys whose Zr or Sn site is substituted by Y or Sb, respectively, and examined their thermoelectric properties considering how the value of VEC affects the conducting property and how the disorder in the material affects the phonon scattering of the material. Such examinations on effects of the substitution may lead to the development of the performance of thermoelectric materials. Recently, Shutoh et al. have reported that Ti_0.5(Zr_0.5Hf_0.5)_0.5NiSn_0.998Sb_0.002 shows the maximum ZT of 1.5 at 700 K [14]. Next, we have also synthesized the double-substituted alloys Zr_1−xY_xNiSn_1−ySb_y and tried to improve the ZT of half-Heusler ZrNiSn system.

Section snippets

Experimental

Samples of various compositions were prepared from the stoichiometric quantities of Zr (99.7% purity), Y (99.9% purity), Ni (99.99% purity), Sn (99.999% purity) and Sb (99.9999% purity) metals by arc melting under argon atmosphere. The button was ground, pressed into pellet and again arc-melted to ensure its homogeneity. The finally obtained button was ground into fine powders and sintered by SPS (spark plasma sintering) apparatus (SCM SPS-511S) at 1373 K for 10 min at a pressure of 40 MPa under a

Zr_1−xY_xNiSn and ZrNiSn_1−ySb_y systems

The X-ray diffraction patterns for the annealed Zr_1−xY_xNiSn with x ≤ 0.10 consisted of the diffraction peaks of ZrNiSn with half-Heusler MgAgAs-type structure. In the ZrNiSn_1−ySb_y system, the X-ray diffraction patterns showing the single phase of half-Heusler type structure was also obtained in the samples with y ≤ 0.10.

Fig. 1(a)–(c) show the temperature and Y content dependence of the electrical resistivity, Seebeck coefficient and thermal conductivity for Zr_1−xY_xNiSn, respectively. The electrical

Summary and conclusion

The conducting properties of half-Heusler ZrNiSn alloys are affected by the VEC, while the lattice contribution to the thermal conductivity, κ_ph, of them is affected by the disorder in the material. We have investigated the effects of the substitution for Zr or Sn site in ZrNiSn by Y or Sb on the thermoelectric properties of it. As for Zr_1−xY_xNiSn system, with increasing x the electrical resistivity increases, while the absolute value of the Seebeck coefficient decreases, and the sign of the

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Thermoelectric properties of half-Heusler alloys Zr1−xYxNiSn1−ySby

Abstract

Introduction

Section snippets

Experimental

Zr1−xYxNiSn and ZrNiSn1−ySby systems

Summary and conclusion

J. Alloy Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

Phys. Rev.

J. Phys.: Condens. Matter

Appl. Phys. Lett.

Thermoelectric properties of half-Heusler alloys Zr_1−xY_xNiSn_1−ySb_y

Zr_1−xY_xNiSn and ZrNiSn_1−ySb_y systems